The invention relates to electronic image methods and devices, and, more particularly, to digital communication and storage systems with compressed images.
Video communication (television, teleconferencing, Internet, digital camera, and so forth) typically transmits a stream of video frames (pictures, images) along with audio over a transmission channel for real time viewing and listening or storage. However, transmission channels frequently add corrupting noise and have limited bandwidth. Consequently, digital video transmission with compression enjoys widespread use. In particular, various standards for compression of digital video have emerged and include the various JPEG and MPEG standards.
Tekalp, Digital Video Processing (Prentice Hall 1995), Clarke, Digital Compression of Still Images and Video (Academic Press 1995), and Schafer et al, Digital Video Coding Standards and Their Role in Video Communications, 83 Proc. IEEE 907 (1995), include summaries of various compression methods, including descriptions of the JPEG, H.261, MPEG-1, and MPEG-2 standards
For encoding a single frame as in JPEG or an I frame in MPEG, Shapiro, Embedded Image Coding Using Zerotrees of Wavelet Coefficients, 41 IEEE Tr. Sig. Proc 3445 (1993) provides a wavelet hierarchical subband decomposition which groups wavelet coefficients at different scales and predicts zero coefficients across scales. This provides a quantization and fully embedded bitstream in the sense that the bitstream of a lower bitrate is embedded in the bitstream of higher bitrates.
Further compression can be achieved by segmenting a picture into subjectively important regions and unimportant regions and then reducing the number of information bits used for the unimportant regions. The segmenting into regions is a preprocessing operation as illustrated in FIG. 1; see Yoshihisa Yamada, Masahide Kaneko, Hiroshi Harashima: “MPEG Encoding Simulator for Enhancing the Image Quality by Manual Operation,” 1996 ITE Annual Convention, 23-5. In particular, functional block A partitions a frame (picture) into and combines regions. The method of extracting regions in an I picture is a manual processing operation, and in P and B pictures, manual processing operations only adjust detail point of results. Functional block B selects a type of preprocessing and sets up parameters for quantization in each region. Functional block C preprocesses by, such as, low pass filtering regions which an operator considers as needless. Several types of filter with different bandwidths are prepared and block C is able to choose one of them according to the importance of the region. Functional block D applies MPEG coding with quantization steps determined in each region.
One aspect of preprocessing by manual operation is that an operator can partition regions as he or she likes; however, the other aspect is that it takes time to operate so that it is not suitable for realtime MPEG encoding and that manual operation may cause inaccurate region segmentation. For example, FIG. 2 shows an image with a center of human which can be considered as subjectively important object. Therefore, we should partition the region between human and his background. However, the region boundary is extremely complex so that it is difficult to partition accurately. Even if the region is partitioned as blocks including its boundaries, it may cause noticeable block distortion. Therefore, it is difficult for this method to preprocess images such as this example.
Hardware and software implementations of JPEG, H.261, MPEG-1, and MPEG-2 compression and decoding exist. Further, programmable microprocessors or digital signal processors, such as the Ultrasparc or TMS320C6xxx, running appropriate software can handle most compression and decoding in real time, and less powerful processors may handle lower bitrate compression and decompression.